Largely ignored because they performed others songs.
The John Lennon interview in Rolling Stone. The thought 3 Dog Night was crap.
That was my era. I couldn’t stand them. Over-produced drivel.
I think you are on to something, there. While 3 Dog Night were obviously very talented, and popular for a good while, they never created a volume of their own compositions that were really unique, broke new ground, etc.
But, then again, the same can be said for quite a few artists who are better remembered.
Maybe another factor is that 3 Dog Night were not, you know, particularly controversial, obnoxious, etc.